grg/compare.txt

   This file is part of GRG 3.2  Copyright (C) 1997 Vadim V. Zhytnikov

   Disclaimer: The opinion expressed here is the opinion of V.Zhytnikov
	       and nobody else.


   GRG 3.2 versus EXCALC

   The GRG 3.2 and EXCALC are two rather similar programs. Both are
based on the computer algebra system REDUCE and designed for the
problems in differential geometry. They work with the differential
forms, vectors, tensors and use convenient notation very similar
to the traditional mathematical one. Both programs work with spaces
of any dimensionality and can represent tensors with respect to
arbitrary frame.

   On the other hand there are also a number of important differences
between EXCALC and GRG 3.2. In particular:

1. EXCALC works with tensors whose components are presented with
   respect to certain frame. GRG 3.2 understands more complicate
   quantities having coordinate, frame, spinorial and enumerating
   indices. GRG 3.2 understands also pseudo-tensors and tensor
   densities. I'd like to emphasize also GRG's ability to work with
   spinors.

2. Working with tensors EXCALC actually knows very little about the
   "covariant" properties of these quantities. On the other hand
   GRG knows all standard covariant operations and operators.
   In particular GRG 3.2 performs frame, spinor and coordinate
   transformations. It automatically computes Lie derivatives,
   covariant derivatives and differentials of any tensor or spinor
   quantity. GRG can easily transform the frame indices to coordinate
   ones and vice versa.

3. GRG 3.2 allows one to save the result of computations in
   the form which can be later used in other computer algebra
   programs: Mathematica, Maple and Macsyma.

4. Unlike EXCALC the GRG 3.2 knows almost 150 built-in quantities
   and numerous built-in formulas for their calculation. So, in
   GRG you have already solutions for many standard problems.
   On the contrary to obtain any result with EXCALC it is
   necessary to write your own program.

5. GRG requires all variables and functions to be declared which
   makes it more reliable than EXCALC.

6. The input languages of GRG and EXCALC are very different.
   EXCALC in fact has no any special language and uses the
   REDUCE programming language with all control instructions:
   loops, if-then-else, procedures etc. GRG uses the completely
   different approach. It has its own quite simple language
   which lacks the aforementioned programming facilities.
   Commands of GRG input language resemble simple English
   phrases. This is especially convenient for people who are
   not interested (or skillful) in programming.

7. The performance of both programs (say the run time for
   analogous problems) is approximately equal.

8. The advantage of EXCALC is that it can operate with abstract
   p-forms while in GRG any p-form is always represented
   as the exterior product of p frame 1-forms (frame may be
   arbitrary).

9. Another potential advantage of EXCALC is the ability to compute
   the variational derivatives. Unfortunately in practice this
   facility is rather limited and buggy.

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